This invention relates to an implantable infusion pump. In particular, it relates to a pump of the type which is programmable to dispense infusate with accordance with different flow rates while operating at positive pressure.
Implantable infusion pumps have been developed to the point of commercial and medical acceptance. Those devices are implantable in the human body and rely on a liquid/vapor equilibrium to maintain constant pressure on the drug which flows through a capillary in order to maintain a constant flow rate. These so called "constant flow" devices are used in a variety of medical applications, for example, to dispense chemotherapy drugs at a relatively constant flow rate. The technology represented by such constant flow devices is reported in Wright, "A Portable Slow Infusion Capsule", 177 J. physiol. 5P (1965) and in U.S. Pat. No. 3,731,681.
In situations where patients require adjustments in the dosage as a function of time constant flow pumps are inadequate. A typical example is diabetes where the quantity of medication, such as insulin, to be infused varies due to requirements of the patient. Fluctuations can occur on a daily basis or more randomly as a function of the ingestion of food. Consequently, to address the shortcomings of constant flow devices and obtain significant flexibility in dosage rates, research has been conducted into so called "implantable programmable" pumps. In the definition of system requirements dealing with such implantable programmable pumps, a device which will provide programmable bolus and basal flow rates over a wide dynamic range is a standing system requirement. This requirement can be set forth in a practical sense by reference to the treatment of diabetes. It is known that the amount of medication, typically insulin, to be infused per unit of time, should be adjusted at certain time intervals. A patient's requirements may fluctuate either at set, know rates or may vary abnormally, for example, by the ingestion of food or by other transitory conditions. Those conditions will call for the administration of a bolus dose of infusate. In the daily administration of insulin, however, the patient may require a basal dose that is supplanted by bolus doses at, for example, meal times. The difference in flow rates between basal and bolus doses may be quite large, in the orders of several times. Thus, a device to achieve proper flow rates over the spectrum of desired rates must have the ability to continuously infuse, at very low flow rates, yet provide, periodically, a substantially increased flow rate. Thus, the design criteria can be summarized as requiring, in the first instance, the ability for continuous, that is basal drug delivery which is adjustable to varying choices of flow rate. The choices should include wide ranges of flow rates to accommodate bolus requirements.
While the basic flow devices offer simplicity in operation and design and as complexity occurs, given the inherent intricacy of programmability, other problems occur such as power consumption, overall system life and failsafe operation.
The art is replete with a number of different implantable programmable pump concepts such as found in U.S. Pat. Nos. 3,894,538; 4,077,405; and 4,443,218. However, the diverse system requirements have not been fulfilled by any existing commercially available or proposed system. Such programmable pumps generally fall into two broad categories. The first are the so called negative pressure pumps which are typified by U.S. Pat. Nos. 4,482,346 and 4,486,190. Both of these prior art devices are solenoid activated negative pressure pumps. A diaphragm storage chamber maintains the drug to be infused in a chamber having a diaphragm which separates the drug from propellant, normally freon, maintained at negative pressure. A solenoid is activated driving an armature and a bellows pumping element. This displacement of the armature opens a check valve which draws drug from the storage chamber into a downstream pumping chamber. A restriction will prevent backflow in the outlet during this short period. When the pump chamber is full, the check valve closes and the solenoid is then de-energized. A spring force typically displaces the bellows into the chamber pumping the drug through a restrictor and into the patient. The bellows armature assembly comes to rest on the check valve to insure no backflow during the rest period, it being noted that the drug chamber pressure is below body pressure. The system operates at negative pressure to insure no forward flow during this rest period.
Negative pressure systems, while offering advantages in terms of accurate dosage and metering, suffer from two significant disadvantages. First, the ingestion of air into the system will stop drug flow. Thus expensive fill and empty apparatus is required for filling and recycling such devices. A more practical, serious problem, exists in the special handling required for negative pressure devices. The drugs used with such devices must be vacuum conditioned, thereby requiring that special steps be taken by those who are, in many cases, technically unsophisticated, and must be packaged and shipped with special care to maintain the vacuum conditioning. Consequently, while such devices offer theoretical technical advantages, in practice they suffer from significant practical disadvantages.
A second class of devices are the so-called positive pressure pumps in combination with an accumulator pump. Such are presented by U.S. Pat. Nos. 4,299,220 and 4,447,224. The device operates at a positive pressure, thereby obviating the problems of negative pressure devices. Given the fact that drug chamber pressure is above body pressure, a remote potential for an overdose of drug exists should all valves in line with the output fail open at the same time. An extremely high degree of safety can be achieved in such systems by redundant or failsafe valves and the addition of sensor/shutdown circuits. However, this means that significant costs are added to the system to address remote possibilities.
Consequently, within the art, a need exists for a reliable programmable pump pressure system that eliminates the problems existing in present designs.
Finally, the device should have the capacity to store in a drug storage reservoir a sufficient amount of infusate to obviate the need for frequent refill. Consequently, within this technology, there exists diverse system requirements which have not been fulfilled by any existing commercially available or proposed system.